THE PRINCIPLES OF VISION 189 



focal point would fall relatively behind the retina. To 

 produce acute vision for all near objects it is necessary 

 to accommodate the vision, which is done unconsciously. 

 For example, take a tripod camera, throw the focusing 

 cloth over your head, and focus an object at 100 feet 

 distance on the ground glass; now, without changing the 

 focusing apparatus, view some object at, say, 10 feet 

 distance from the camera, and you v/ill notice the image 

 on the ground glass is blurred. Now rack the lens 

 forward, increasing the distance from the lens to the 

 ground glass, and the real image will appear sharply 

 cut in detail. This is called focusing the object. Ac- 

 commodation of vision is practically the same thing, 

 except it is done in a different way and by a physiologic 

 organ instead of a physical apparatus. Accommodation 

 is accomplished not by increasing the distance between 

 the lens and the retina, but by increasing the convexity 

 of the crystalline lens sufficiently to cause a clearly 

 defined retinal image. The ciliary muscle is the governor 

 controlling the variations in the convexity of the crystal- 

 line lens for all distances within 20 feet. When the 

 ciliary muscle contracts, the zonule of Zinn, which 

 supports the lens to the muscle, relaxes, allowing the 

 lens in its capsule to expand and become more convex 

 by its own elasticity. The nearer the object to the eye 

 the greater must be the accommodation. 



In young subjects accommodation is very easily ac- 

 compHshed because the lens is soft and very readily re- 



